1. Field of the Invention
The present invention relates to an apparatus and method for measurement in a lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Lithography requires the projection of patterns onto a substrate with a high degree of accuracy. In order to ensure that projection is achieved at this degree of accuracy, the lithographic apparatus may perform various calibration measurements, and in some instances, the lithographic apparatus may be adjusted in response to these measurements.
Conventional lithographic apparatus often use transmissive reticles. However, in some proposed lithographic apparatus, the transmissive reticles have been replaced with arrays of individually controllable elements, such as mirrors. In these non-conventional lithographic apparatus, conventional calibration measurement systems and methods may no longer be suitable or may no longer be optimal.
Therefore, what is needed is a system and method that measure interference of radiation beams projected by projection systems within lithographic apparatus using arrays of individually controllable elements.